The present invention relates to a liquid level detecting apparatus including a liquid level sensor of the variable resistance type which is to be disposed in a tank, and more particularly to a liquid level detecting apparatus which is suitable for detecting a remaining amount of a fuel containing alcohol stored in a fuel tank of a vehicle.
In a fuel tank of a vehicle which uses a liquid such as gasoline or light oil as a fuel, a liquid level sensor which detects a remaining amount of the fuel is disposed. As a liquid level sensor of this type, a sensor shown in FIGS. 5 and 6 is known (for example, see Patent Reference 1).
In the liquid level sensor 101 disclosed in Patent Reference 1, as shown in FIG. 5, a floats 103 is attached to the leading end of an arm 102, and the basal end side of the arm 102 is swingably supported by a frame 104. On the frame 104, a wiring board 105, and a sliding arm 106 which slides on the wiring board 105 in conjunction with the arm 102 are disposed.
As shown in FIG. 6, a first sliding portion 107 and a second sliding portion 110 are provided on the wiring board 105. The first sliding portion 107 includes plural first conductive segments 108 which are vertically arranged at intervals and a resistor 109 to which the plural first conductive segments 108 are connected. The second sliding portion 110 includes plural second conductive segments 111 which are vertically arranged at intervals and a conductive coupling portion 112 to which the plural second conductive segments 111 are connected. First and second contacts 113, 114 which are contacted with the first and second conductive segments 108, 111, respectively, and which are electrically connected to each other are disposed on the sliding arm 106.
When the arm 102 is swung in accordance with a displacement of the liquid level, and the sliding arm 106 is swung in conjunction with the swing, both the first conductive segment 108 with which the first contact 113 is contacted, and the second conductive segment 111 with which the second contact 114 is contacted are changed, so that the total length of the resistor 109 which is in the circuit between a connection land 115 of the first sliding portion 107 and a connection land 116 of the second sliding portion 110 is increased or decreased.
The connection land 115 of the first sliding portion 107 is connected to the plus side of a power supply system, and the connection land 116 of the second sliding portion 110 is connected to the minus (ground) side of the power supply system. When the circuit between the connection lands 115, 116 are energized, a detection signal (a resistance or a voltage) corresponding to a change of the resistance between the connection lands 115, 116 is output.
In the thus configured liquid level sensor 101, the rise and fall of the liquid level, and the increase and decrease of the resistance exhibit the same tendency. When the liquid level is raised, namely, the sliding arm 106 is swung toward the upper side in FIG. 6, and the total length of the resistor 109 which is in the circuit between the connection lands 115, 116 is decreased, whereby the resistance of the circuit between the connection lands 115, 116 is decreased.
Conversely, when the liquid level is lowered, the sliding arm 106 is swung toward the lower side in FIG. 6, and the total length of the resistor 109 which is in the circuit between the connection lands 115, 116 is increased, whereby the resistance of the circuit between the connection lands 115, 116 is increased. Then, the liquid level is detected on the basis of the change of the resistance. In FIG. 6, point F indicates the swung position of the sliding arm 106 in the case where the liquid level is at the full position, and point E indicates the swung position of the sliding arm 106 in the case where the liquid level is at the empty position.
In a liquid level detecting apparatus comprising the liquid level sensor 101 of the variable resistance type, the liquid level sensor 101 is always energized, a controlling device (for example, a microcomputer) samples the detection signal of the liquid level sensor 101 at regular intervals, and controls a fuel meter to display the remaining amount.
Recently, techniques in which alcohol (ethanol or methanol) is mixed with a main fuel such as gasoline or light oil, or in which alcohol itself is used as a main fuel are studied. Alcohol is an electrolyte solution. When a liquid level sensor of the variable resistance type is immersed in an electrolyte solution, there is a possibility that electrolytic corrosion occurs, and metals of the plus electrode (conductor) are eluted with the fuel by electrolysis, so that the resistance of the conductor is increased. By contrast, plus ions in the fuel precipitate on the minus electrode (conductor), and the contact resistance with respect to the sliding contact is increased. Therefore, a conventional liquid level sensor such as the above-described liquid level sensor 101 of the variable resistance type tends to cause a failure in indication of the remaining amount.
In order to suppress the influences of electrolytic corrosion, it is effective to shorten the energization time of the liquid level sensor. Patent Reference 2 and 3 disclose examples of a technique in which the energization time is shortened.
For example, Patent Reference 2 discloses a liquid level detecting apparatus comprising: a liquid level detecting portion which includes at least partly an electrolytic corrosive member, and which is disposed in a liquid storage tank to detect the level of a liquid; and a controlling portion for controlling the liquid level detecting portion to intermittently operate (operate only during a predetermined time at each predetermined period).
Patent Reference 3 discloses an apparatus in which a switching circuit for controlling energization is disposed in a power supply circuit, and a liquid level sensor is intermittently energized at a constant period.
[Patent Reference 1] JP-A-2003-65827
[Patent Reference 2] JP-A-2006-214828
[Patent Reference 3] JP-A-2002-214023
However, although the liquid level sensor is intermittently energized, both the conventional apparatuses disclosed in Patent References 2 and 3 are not configured so that the energization time is changed according to the situation. Depending on the manner of setting the energization time, there still remain possibilities that the accuracy of detecting the liquid amount is lowered, and that a high effect of preventing electrolytic corrosion from occurring cannot be attained.